In many laser applications (for example, chemical sensing), high-intensity laser light is needed. One way to provide high-intensity light is using light trapped inside an optical cavity. An optical cavity or resonator consists of two or more mirrored surfaces arranged so that incident light may be trapped, bouncing back and forth between the mirrors. In this way, the light inside the cavity may be many orders of magnitude more intense than the incident light.
In many applications, the optical gain medium (such as a helium neon discharge tube) is within the optical cavity. For a typical diode laser, the cavity mirrors are deposited directly on the diode gain medium itself. For some applications, however, such as frequency-tuning and linewidth-narrowing, one or both of the diode's facets is antireflection-coated and the diode is operated inside an optical cavity defined by mirrors external to the diode. While a diode gain media may be operated inside such a cavity, the low damage threshold of the diode's emission facet severely limits the amount of power build-up in the cavity.
To overcome this limitation while still generating a large optical field, the diode laser may be placed outside of a separate high-finesse optical cavity in which the diode laser radiation is trapped. This separate cavity is referred to hereinafter as a "build-up" cavity. Diode lasers, however, emit radiation with an optical bandwidth that is much larger than that of a high-finesse build-up cavity. To achieve substantial amplification of diode laser radiation in a build-up cavity the diode laser must be forced to emit coherent radiation with a bandwidth that approaches or matches that of the cavity at a cavity resonant frequency. This process is hereinafter called "optical locking."
One way to reduce the bandwidth of diode lasers is to use all-electronic frequency-locking of diode lasers. This technique, however, requires very fast servos, a large degree of optical isolation of the diode laser from the cavity, and sophisticated electronic control.
As an alternative, substantial linewidth reduction can be achieved with optical feedback (i.e., passive) schemes. For example, Dahmani et al., in "Frequency stabilization of semi-conductor lasers by resonant optical feedback," Opt. Lett., 12, pp. 876-878 (1987), reported passive optical locking of a diode laser to a build-up cavity. In this technique, light from a diode laser is directed into a build-up cavity. If the light has a frequency matched to a cavity resonance frequency, the light is trapped. A portion of the trapped light is then directed back into the diode laser to act as a passive feedback mechanism, which locks the frequency of the low-finesse diode laser to that of the high-finesse build-up cavity, as well as reduces the diode emission bandwidth.
A shortcoming of systems similar to that of Dahmani et al. is that such systems employ weak optical locking: only a very minute portion of the light in the build-up cavity is fed back to the diode laser. The disadvantage of the weak optical locking technique is that it still requires careful electro-mechanical control of both the magnitude and phase of the light fed back to the diode laser. Additionally, such systems contain at least four reflectors.
Passive all-optical locking of antireflection-coated diode lasers to external resonant cavities has recently been exploited extensively. Examples include frequency doubling (W. Lenth and W. P. Risk in U.S. Pat. No. 5,038,352, "Laser system and method using a nonlinear crystal resonator," August 1991; W. J. Kozlovsky et al., "Blue light generation by resonator-enhanced frequency doubling of an extended-cavity diode laser," August 1994, vol. 65(5), pp. 525-527, Appl. Phys. Lett.), frequency mixing (P. G. Wigley, Q. Zhang, E. Miesak, and G. J. Dixon, "High power 467 nm passively-locked signal-resonant sum frequency laser," Post Deadline Paper CPD21-1, Conference on Lasers and Electro-optics, Baltimore, Md., Optical Society of America, 1995), and chemical sensing (David A. King, et al., in U.S. Pat. No. 5,432,610, "Diode-pumped power build-up cavity for chemical sensing," July, 1995). King et al., (supra and incorporated by reference in its entirety herein) describe several embodiments in which a diode laser is optically locked to an external resonant cavity. King et al. teach that there is a broad restriction on the diode current and additional components may be required to eliminate off-resonance reflections for a system containing three reflective elements.
To illustrate the difficulty of passive all-optical locking of diode laser, a brief description of the physics of an optical cavity is given in the following. As depicted in FIG. 1, two reflective surfaces 2 and 4 (with reflectivities (reflection coefficients) R.sub.1 and R.sub.2 respectively) define a cavity 6. This cavity 6 has a comb of resonant frequencies where the comb spacing is c/2L (c is the speed of light in the cavity and L is the optical distance between the two reflective surfaces 2 and 4).
Light incident on a linear cavity generally undergoes one of two possible phenomena as depicted in FIG. 1. In FIG. 1A, the frequency of the incident light 8 is far from a cavity resonance frequency. Thus, the incident light 8 is simply reflected as reflected light 10 by surface 2. FIG. 1B depicts the situation when the incident light 8 is at (or very near) a cavity resonant frequency. In this case, the incident light is trapped as an intracavity beam 12 between surfaces 2 and 4. The trapped light additionally leaks through surfaces 2 and 4, affecting the reflected beam 10 and the transmitted beam 14 from the cavity respectively. The leakage is out of phase with the incident beam 8, thus causing a destructive interference with the portion of beam 10 that is simply and non-resonantly reflected from surface 2.
When the incident beam 8 is at a cavity resonant frequency, the effective reflectivity (reflection coeffieient) of the cavity 6 is lower than the simple nonresonant reflectivity (or reflection coefficient) of surface 2. This effect is shown in FIG. 1C, in which the reflectivity of the cavity (I.sub.ref /I.sub.inc) shown in FIG. 1A and FIG. 1B is plotted as a function of normalized frequency. The frequency is normalized to a comb spacing of the cavity such that a cavity resonance occurs for each integral value of normalized frequency. The cavity bandwidth is the full width at half maximum of each resonance and becomes smaller as the reflectivities of surfaces 2 and 4 decrease. When R.sub.1 equals R.sub.2, the magnitude of the resonant and nonresonant reflections from surface 2 are equal and their phases differ by 180.degree.. In this way, the cavity reflectivity drops to zero (in the absence of scattering) on a cavity resonance.
The goal of all-optical locking of a diode laser to a cavity is to generate intracavity beam 12 with incident beam 8 from the diode laser. This imposes desirable optical properties (for example, bandwidth and frequency) that originate from the cavity on the diode laser. The reflected beam 10 from the cavity is used to frequency-lock the diode laser to a cavity resonance. However, FIG. 1C shows that the reflected beam 10 is the weakest at a cavity resonance. Thus, it appears that, by optical feedback, as the diode current is increased, the laser tends to reach threshold at a frequency other than a cavity resonance frequency. Therefore, it has long been believed by those skilled in the art that the structure shown in FIG. 1A is highly unsuitable for frequency-locking of a diode laser.
Various approaches have been used to reduce the destructive interference mentioned above and to ensure that the most intense reflection back into the diode laser originates uniquely from the optical cavity. A simple approach is to use additional cavity reflectors or reflections that allows spatial isolation of the resonant feedback (Dahmani et al., "Frequency stabilization of semiconductor lasers by resonant optical feedback," supra). Other solutions rely on using very small feedback into the diode laser from mirror-induced birefringence (C. E. Tanner, et al., "Atomic beam collimation using a laser diode with a self locking power-build-up cavity," May 1988, vol. 13 (5), pp. 357-359, Optics Letters) or very weakly excited counter-propagating modes (A. Hemmerich et al., "Second-harmonic generation and optical stabilization of a diode laser in an external ring resonator," April 1990, Vol. 15 (7), pp. 372-374, Optics Letters). All these solutions require additional components that tend to increase the complexity and expense of constructing the laser system. What is needed is a passively locked laser with relatively simple structure and yet capable of generating high-intensity light.